how long is a piece of string corecing String

playground.rs

use rand; // 0.7.3
use rand::Rng;

fn make_a_string() -> String {
    let mut s = String::new();
    s.push_str("Hello World");

    let mut rng = rand::thread_rng();
    let r = rng.gen_range(0, 100);
    for _ in 0..r {
        s.push_str("!");
    }
    s
}

fn make_a_string_with(string: &str) -> String {
    let mut s = String::new();
    s.push_str(string);

    let mut rng = rand::thread_rng();
    let r = rng.gen_range(0, 100);
    for _ in 0..r {
        s.push_str("!");
    }
    s
}

fn main() {
    // how long is a piece of string
    let string: String = make_a_string();
    let s = make_a_string_with(&string); // compiler error without &
    println!("len: {}, string: {}", s.len(), s)
}